1. Field of the Invention
The present invention relates to a slotted antenna cover for protection of the RF antenna of a nuclear magnetic resonance (NMR) tool.
2. Background of the Related Art
To obtain hydrocarbons such as oil and gas, a drilling assembly (also referred to as the “bottom hole assembly” or the “BHA”) carrying a drill bit at its bottom end is conveyed into the well bore or borehole. The drilling assembly is usually conveyed into the well bore by a coiled-tubing or a drill pipe. In the case of the coiled-tubing, the drill bit is rotated by a drilling motor or “mud motor” which provides rotational force when a drilling fluid is pumped from the surface into the coiled-tubing. In the case of the drill pipe, it is rotated by a power source (usually an electric motor) at the surface, which rotates the drill pipe and thus the drill bit.
Bottom hole assemblies (“BHA”) generally include several formation evaluation sensors for determining various parameters of the formation surrounding the BHA during the drilling of the well bore. Such sensors are usually referred to as the measurement-while-drilling (“MWD”) sensors. Sensors are also deployed after the borehole drilling has been completed. Depending a sensory device down hole via a wire line performs such operations.
Such sensors, whether MWD or wire line, have traditionally utilized electromagnetic propagation sensors for measuring the resistivity, dielectric constant, water saturation of the formation, and nuclear sensors for determining the porosity of the formation and acoustic sensors to determine the formation acoustic velocity and porosity. Other down hole sensors that have been used include sensors for determining the formation density and permeability. The bottom hole assemblies also include devices to determine the BHA inclination and azimuth, as well as pressure sensors, temperature sensors, gamma ray devices, and devices that aid in orienting the drill bit in a particular direction and to change the drilling direction. Acoustic and resistivity devices have been proposed for determining bed boundaries around and in some cases in front of the drill bit. NMR sensors as MWD sensors as well as wire line sensors can provide direct measurement for porosity, water saturation and indirect measurements for permeability and other formation parameters of interest.
NMR sensors utilize permanent magnets to generate a static magnetic field, B0 in a formation surrounding the borehole in which the MWD or wire line tool is deployed. Typically a radio frequency (RF) solenoid coil is disposed between the permanent magnets or around the magnets to induce an RF magnetic field into the formation. The magnets and the RF coils are positioned so that the static magnetic field B0 and the RF field occur perpendicular to each other in at least a portion of the formation surrounding the bore hole and the NMR tool. In the region of interest, or region of investigation, where the RF and B0 fields are perpendicular to each other, NMR measurements are made to determine parameters of interest for the surrounding formation.
In MWD operations, NMR sensors can be located inside and outside of a drill collar for performing measurements on the formation and its fluid content. A conventional MWD drill collar comprises a metallic structure that conveys rotational torque required during drilling operations. Moreover, the drill collar provides a hollow center section that provides a conduit for the drilling fluid or drilling mud that is used to lubricate the drill bit and carry the drilled cuttings from the borehole to the surface. Since NMR radio frequency electromagnetic fields do not penetrate the metallic body of the drill collar, electromagnetic field sensors typically are mounted outside of the metallic drill collar body. Because these NMR sensors are on the outside of the drilling collar, they are exposed to the abrasive rock in the formation during drilling operations and are thus subject to abrasion and wear resulting from particles in the drilling mud and the impact of the sensor against the earth formation during drilling.
A typical MWD tool is described in EP-A-0581666 (Kleinberg). The MWD tool comprises a tubular drill collar, a drill head positioned at an axial end of the drill collar, and an NMR sensor. The NMR sensor comprises a pair of tubular main magnets, which generate a static (B0) magnetic field, each of which is located in an internal recess of the drill collar. The Kleinberg tool provides an RF antenna located in an external recess in the drill collar between the main magnets. The RF antenna recess is optionally filled with a magnetically soft ferrite to improve the efficiency of the antenna.
U.S. Pat. No. 6,288,548 discloses a slotted metal tubular having axial slots to allow inward and outward passage of electromagnetic fields for resistivity measurements. This configuration is too lossy to be used with NMR sensors due to the production of eddy currents.
Known down hole NMR tools use resonating antennas for radiating RF electromagnetic NMR pulses and/or receiving alternating magnetic fields at the resonance frequency of the detected NMR. Typically the NMRantenna is a simple solenoid coil in combination with an attached capacitor to form a resonating circuit. The typical NMRantenna is protected against wear and deterioration or failure due to the abrasive effects on the antenna from exposure to the formation during drilling operations. The protection is effected by a cover made from ceramics, rubber, epoxy or other electrically non-conductive material. All these materials have major disadvantages. They are either brittle (ceramic) or soft. Thus, there is a need for an NMR antenna cover with better mechanical robustness. Therefore there is a need for a NMR antenna cover, made from tough metal, that does not significantly reduce efficiency of the antenna through the production of eddy currents.